I like No.5 best too, followed by No.2. The only quibble I have is using a different colour for the US, this makes it look too much like US as in America and might give the impression it's just an American organisation & not worldwide.But as no one else seems to have picked up on this, maybe I'm just nit picking :-)

I use to like the simplicity in logos, but in thi case i thin is very important to see an element related as the atom. Just maybe a little retouch to maked more solid, i mean in order to be visible in a miniature for example or or to be recognised at a first sight... One more time... EXCUSE MY ENGLISH.. im from Venezuela